FIG. 7 shows the construction of a conventional quadrature demodulator. Conventional quadrature demodulators have been constructed to form a feedback loop arrangement. In-phase (I) component (a), and quadrature (Q) component (b) of the digital modulated signal are input to the quadrature demodulator via input terminals 101 and 103.
These signals are inputted to A/D converters 105 and 107 and converted to digital values. The outputs of the A/Ds 105 and 107, respectively, are inputted to a complex multiplier 109. The digitized in-phase and quadrature components are demodulated at a complex multiplier 109 by using a local carrier which is introduced separately into the complex multiplier 109. Then, a frequency error and a phase error are removed from the demodulated signals by controlling the local carrier (to be described later) in accordance with feedback operation.
The calculation of the complex multiplier is input to roll-off filters 111 and 113, where they are shaped by being subjected to a filtering process. Then the shaped I and Q components (c) and (d) are output from the circuit through output terminals 137 and 139.
The calculation of the complex multiplier is also supplied to a convolution circuit 19. The values of signals (c) and (d) designate a coordinate on an I-Q orthogonal signal constellation. The designated coordinate will be referred to as a symbol. If a symbol of a subject signal matches a reference symbol, such a match means that no errors exist in the frequencies and phases of the subject signal. However, if the symbol deviates from the reference symbol, then, an error does exist. As a result of the error of a phase angle between the I-Q coordinate of the symbol and the reference symbol coordinate is detected by a detector 121.
The convolution circuit 119 is provided before the phase detector 121 for rotating the symbols a sufficient number of rotations so that the symbols are positioned in the first quadrant of the I-Q plane. The phase detector 121 detects the phase angle between the symbol and the reference symbol.
A phase angle signal (e) detected at the phase detector 121 is supplied to an error detector 123 and a selector 127. The differentiator 123 detects the error between the phases of sequential signals by calculating the difference between the input signals. Then, the differentiator 123 outputs a symbol to a sign discriminator 125, which determines whether the sign of the symbol is positive and negative.
The selector 127 selects either the phase angle signal (e) of the phase detector 121 or the sign signal (f) obtained in the sign discriminator 125. The signal selection at the selector 127 is organized by a selector controller 129. When the quadrature demodulator executes a frequency synchronizing operation the selector 127 selects the sign signal (f) from the sign discriminator 125. On the other hand, when the quadrature demodulator executes a phase synchronizing operation the selector 127 selects the phase angle signal (e) directly supplied from the phase detector 121.
The gain of the signal selected at the selector 127 is properly adjusted in a gain adjuster 131. Then, the signal is smoothed in a low pass filter (LPF) 133 to remove its high frequency. The smoothed signal of the LPF 133 is used as a control signal to control the local carrier generated at a numerically controlled oscillator (NCO) 135. The local carrier is then converted into a cosine wave component and a sine wave component at a cosine converter 115 and a sine converter 117, respectively. The cosine and the sine wave components are then multiplied with the in-phase and the quadrature components of the modulation signal at the complex multiplier 109, in order to produce baseband signals, i.e., demodulated in-phase and quadrature components.
The feedback loop of the conventional quadrature demodulator, as shown in FIG. 7, disadvantageously has a loop delay. The loop delay represents the time that it takes for the signals input to travel through the feedback loop and return back to the input terminal. In order words, the time necessary for the input signal supplied to the complex multiplier 109 to leave and return back to the complex multiplier 109 via the feedback loop.
Since the quadrature demodulator for the digital demodulated signals digitally processes the signals, latches constructed by, for example, flip-flops circuits are added in many circuit elements for synchronizing signals among the circuit's elements.
Latches are indispensable components for all circuit elements. However, as the number of latches added to a circuit increases, the more likely the loop delay will increase. Further, the feedback loop includes additional circuits, which perform less important functions in the frequency or phase locked loops, such as the roll-off filters 111 and 113. But these additional circuits also require latches. As a result, the loop delay of the feedback loop increases even further due to the increased number of latches.
If the loop delay of the feedback loop continues to increase, at some point, the circuit will begin to experience drawbacks, wherein the frequency pull-in range of the feedback loop will be reduced, and the phase jitter generated after the phase synchronization increases. By definition, the frequency pull-in range is the measure of the maximum reference frequency deviation from the nominal rate that can be overcome by a slave clock to pull itself into synchronization, and the phase jitters are short-time variations of the significant instants of a digital signal from its ideal position in time. Specifically, in the conventional circuit, the frequency pull-in range will begin to vary in accordance with the loop delay, which increases according to the number of latches. As a result, there is a need to decrease the number latches in order to improve the circuit's ability to efficiently demodulating the incoming signals.